1. Field of the Invention
This invention relates generally to star couplers for routing optical signals through optical communication systems. More specifically, the invention relates to optical coupler combinations having overlapped star couplers with at least one input waveguide and at least one output waveguide for guiding optical signals to and through the coupler combination.
2. Description of the Related Art
Photonic Lightguide Circuits (PLCs) are components used in optical communication systems for routing optical signals through the systems to enable optical communication. PLCs typically comprise substrate materials, typically glass or silicon but oftentimes other more exotic materials. Lithographic means and processes are used to produce optical structures and subcomponents such as waveguides near the surface of the substrate. Light is guided near the surface region of the substrate where optical signals can be routed through the PLC. The optical subcomponents on the PLCs optically process the light traversing the surface of the substrate so that the PLC can perform the particular function for which it is designed. One such optical component is an optical N.times.M coupler. In the past, optical couplers have taken up a large amount of "real estate" in the PLC and have also been limited with respect to the number of channels that can be routed through the couplers since they are physically less able to handle many input waveguides having different channel frequencies. Thus, current optical couplers for PLCs are not optimally functional for use in modern, dense wavelength division multiplexed (DWDM) optical communication systems.
It has been known to place two N.times.M star couplers in the PLC to provide stable, low-loss connections between them, particularly in Dragone routers which are a well-known type of PLC. The N.times.M star couplers are made by N input waveguides arrayed along a circular arc that is approximately centered about a second arc. On the second arc M output waveguides are approximately centered about the central waveguide end of the first circular arc. Personnel of the assignee of the present invention have developed a wafer having two overlapped star couplers to create two Dragone routers each consisting of a pair of star couplers on a single wafer, and wherein the Dragone router on the wafer with the best performance is utilized while the other is discarded or ignored. This overlapping of couplers did not, however, produce a PLC wherein the overlapped couplers could be used simultaneously and instead the couplers were overlapped merely to maximize the probability that at least one highly functional Dragone router would be created during the fabrication process.
Other methods have been proposed for producing a plurality of working routers by folding or overlapping routers to save space on a wafer. See T. Brenner et al., Electronics Letters, Vol. 32, 18 (Aug. 29, 1996). However, the couplers of Brenner et al. are not intended to be used simultaneously but, rather, are designed to produce multiple routers that are then individually cleaved or diced from the wafer to produce a plurality of separate PLCs with independent star couplers.
There exists a long-felt need in the art, recognized by the inventors of the present invention, for PLCs that include overlapped N.times.M star couplers so that optical signals in the PLC can be routed over one another without interfering with each other. Furthermore, PLCs of this type should maximize the utilization of space on a wafer, thereby conserving and better utilizing the available real estate thereon. Moreover, the use of overlapped N.times.M star couplers in PLCs should promote the capability to make more compact DWDM routers with smaller inter-channel wavelength spacings. Such needs have not heretofore been satisfied or achieved in the art.